Patent application WO2010/116182 describes a motor drive circuit for an EPS having a positive and a negative supply rail for connection to a battery. The motor drive circuit comprises a plurality of motor drive sub circuits which each selectively permit current to flow into or out of a respective phase of a multi-phase motor in response to control signals from a motor control circuit and a switching means comprising transistor switches which are in series with a respective phase of the motor which are normally closed to permit the flow of current to and from the sub circuit to the respective motor phase. A fault signal detector detects at least one fault condition and in the event of a fault condition being detected causes switches to open. A snubber circuit is associated with the motor and is arranged so that following the opening of the switch energy stored in the motor windings is diverted away from the switching means through the snubber circuit to the battery.
A problem of such motor drive control is that the transistor switches cause unwanted power losses and increase the cost price and the complexity of the motor drive circuitry. Alternatively using a traditional star point relay having switches to disconnect the phases increases cost and failure rate.
However avoiding the switches may cause further problems. United States Patent document U.S. Pat. No. 8,049,363 describes such problems that may occur in an EPS system when faults occur in the motor drive sub circuits, for example when a short circuit occurs across a drive stage transistor. Without means to isolate the motor from the motor drive stages, the phase windings are still connected together and also still connected to the drive stage. As a result of this an electrically conducting path could still be present around the bridge (e.g. through a short circuited transistor and two phase windings). Then, on rotation of the motor shaft drive due to rotation of the steering column, a back EMF is produced in one or more of the windings. Due the back EMF and the complete electrical path around the bridge, a substantial current can flow through the motor phase windings, which produces a torque in the opposite sense to the rotation of the motor shaft. This resisting (counteracting) torque is highly undesirable because in this case, not only is steering assistance lost due to disconnection of the power supply, but a resisting torque is then applied which makes turning the steering column difficult. These problems are, of course, not unique to applications in electric power assisted steering systems.